The role of apolipoprotein-B-containing lipoproteins in hyperlipidemias and atherosclerosis-coronary heart disease make their metabolism of central concern to public health. We, and others, have recently shown that apolipoprotein-B (apoB) is heterogeneous, encompassing both the peptide (PI) in human VLDL-LDL and a novel peptide (PIII), Mr 200,000) which may be fragment of PI, and is the major component in human and rat chylomicrons, and in rat VLDL. We have shown in the rat that PIII-containing lipoproteins are moe rapidly metabolized than those which contain PI, and that this explains the 20-fold decrease in PIII/PI ratio from VLDL to LDL in this species. In light of the previously unrecognized existence of two distinct apoB peptides, we propose to compare their roles in the synthesis, secretion, plasma turnover and eventual removal of VLDL/chylomicrons. We ask the following questions: Is the PIII a cleavage product of PI, or are the two translated from different messenger RNA? Are the rates of PI/PIII synthesis regulated by triglyceride (TG) output from liver and intestine? Are all 2poB peptides productively assembled into nascent VLDL, and how does assembly occur? Once secreted, what are the relative rate and extent of TG hydrolysis of PI- and PIII-containing particles before they become recognized as remnants by the liver? What are the determinants for this recognition? What are the relative rates of reminant formation and removal for PI- vs. PIII-containing VLDL? We approach these questions by tracer kinetic studies of both the intracellular and intravascular aspects of apoB metabolism, using immune precipitation/SDS-polyacrylamide gel electrophoresis to separate PI and PIII. We further propose to study the interactions of PI- vs. PIII-containing lipoproteins with hepatic apoB/apoE receptors in vitro, and to characterize the molecular properties of the receptor. We will also continue structural studies on apoB-PI and -PIII to better understand their interaction with other surface and core constituents in chylomicrons, VLDL and LDL.